The present invention relates to a method for knocking off a web wound on a turret winder according to the preamble of claim 1 and to a turret winder according to the preamble of claim 6.
Such turret winders are employed for example in rotary printing machines as coil-forming winders for a printed web, as is shown for example in EP 1 454 858 A1. The web is wound onto a core against which a pressure roller of a knock-off device has been set in order to assure a uniform and blister-free winding of the winding layers. While the pressure roller rotates freely, the coil that is held by a chuck is driven for rotation by a rotary drive system. By means of an adjustable support of the knock-off device, e. g, a carriage driven by a pneumatic cylinder, the position of the pressure roller may be adjusted in accordance with the radius of the coil.
The coils of the turret winder are either disposed at the ends of roller arms that project from a central pivoting axle, or the winder is a so-called rotary star winder wherein the coils are arranged at the periphery of a rotary star and are pivotable by means of a central rotary drive system for the rotary star. A change of the coil that engages the pressure roller of the knock-off device can be accomplished by rotating the rotary star. When, for example, a completed coil in the winding position at the pressure roller shall be replaced by an empty coil or core, the completed coil pivots into a withdrawal position during continued transport of the web, while the empty core simultaneously reaches the winding position. In this position, the knock-off operation may be performed, wherein the running web is pressed against an adhesive portion of the new core and, at the same time, the web is cut, so that the web may directly run onto the new coil via the pressure roller, while the completed coil with the cut-off web can be withdrawn. In conventional turret winders, the knock-off operation is performed by advancing the knock-off device together with the pressure roller, so that the latter presses the web against the adhesive portion on the core which will then take over the web, while a knock-off blade abruptly cuts through the web in a suitable position downstream of the adhesive portion. This knock-off operation must be performed very quickly, in accordance with the speed of the running web. However, when the pressure roller is advanced with high speed, which results in a hard impact on the winding core, problems may be caused which are due to a swing-back of the pressure roller, vibrations and the like. It may therefore happen that the process of knocking off the web and transferring the same onto the new coil fails.